A direct touch type metal diaphragm valve (hereinafter called a “metal diaphragm valve”) is generally structured as shown in FIG. 10. It is not only excellent in responsitivity and replacementabity of fluids, but also is exhibits near-particle free characteristics. Therefore, the metal diaphragm valve has been widely used in the fields such as facilities for semiconductor manufacturing, chemical goods manufacturing, food products manufacturing, and the like.
According to FIG. 10, 21 designates a body, 22 designates a metal diaphragm, 23 designates a stopper mechanism, 24 designates a bonnet, 25 designates a bonnet nut, 26 designates a disc, 27 designates a diaphragm presser, 28 designates a stem, 29 designates a handle, 30, 31 designate a fluid inlet and outlet, 32 designates a valve chamber and 33 designates a valve seat. A fluid passage is closed by means of a metal diaphragm 22 that is pressed from above to a valve seat 33 through the mediation of the diaphragm presser 27, while the fluid passage is opened by means of the diaphragm presser 27 that is pulled upward to restore the metal diaphragm 22 to its original reverse-dish shape. A detailed description is omitted herewith because this type of metal diaphragm has been disclosed by others (e.g. TOKU-KAI-HEI No. 5-80858, and the like).
The afore-mentioned metal diaphragm 22 is normally formed with a laminated body made out of 2 to 3 pieces of 0.1 to 0.2 mm thick stainless steel made plate, and is made by means of a bulging mould in a reverse-dish shape in the center of the laminated body cut in a round shape. The maximum height Δh of the bulging portion of the reversed-dish shaped metal diaphragm 22 is set at approximately 1.1 to 1.3 mm with the afore-mentioned 9.52 mmφ metal diaphragm (outer diameter of approximately 26 mmφ).
The afore-mentioned valve seat 33 has been manufactured by forming it in a desired shape using engineering plastics (e.g. PFA: perfluoroalkoxyi), and fitting it into a valve seat holding groove integrally formed with the valve body 21, thus the valve seat is fixed by partially clapping it to the holding groove.
Thus, it is necessarily required that a valve stoke ΔS (namely, a deformation volume of the metal diaphragm 22) is made to be large in order to increase the flow volume of gases passing through the fluid passage because, as shown in FIG. 10, the reverse-shaped metal diaphragm 22 is employed for this type of metal diaphragm valve. Accordingly, the valve is so made that a maximum bulge height Δh of the metal diaphragm 22, which is normally bulge-deformed in the reverse-shape, is made large, and the distance nearly equivalent to the height Δh is made to be a full stroke ΔS of the valve.
As a result, the metal diaphragm 22 is deformed, by size close to the bulge height Δh, with thrust when the valve is closed in the case of the NO (normally open) type metal diaphragm so that the metal diaphragm is pressed to a valve seat 33 so as to be in a near-flat shaped configuration. It is also same with the NC (normally closed) type metal diaphragm valve. The metal diaphragm 22 is deformed, by size close to the maximum bulge height Δh, at all times with thrust so that the diaphragm becomes near-flat shaped, and is restored to its original bulged reverse-dish shape due to elastic force and fluid pressure of the metal diaphragm 22 when the valve opens.
On the other hand, as stated above, the maximum flow rate of this type of metal diaphragm valve is closely related to the valve stroke ΔS of the metal diaphragm 22, thus making it possible that the greater flow rate of fluids is chosen by making the valve stroke ΔS larger. However, as stated above, the elastic deformation volume of the metal diaphragm has its own limitation. Therefore, normally, with the metal diaphragm 22 (having an outer diameter of 26 mmφ) of a valve having an inside diameter of 9.52 mmφ of the fluid passage, the maximum bulge height Δ is limited to approximately 1.2 to 1.3 mm because the larger a maximum bulge height Δh becomes, the more cracks and the like are likely to be caused due to the deformation of the metal diaphragm 22.
As disclosed above, the relation between the valve stroke ΔS of the afore-mentioned metal diaphragm valve and the flow rate and the like is generally indicated by a flow rate coefficient (Cv value). More specifically, the definition of the Cv value of the afore-mentioned valve is “a numeric value of the flow rate expressed in gal/min when clean water flows while keeping the differential pressure of the valve inlet and outlet at 1 psi.” When the fluid is water, the Cv value is derived from:[mathematical expression 1]Q′=Cv√{square root over ((p1′−p2′)/Gl)}′  Equation (1)where Q′=the flow rate gal/min, P1′=inlet pressure psi, Gl′ is the specific gravity of the fluid (when water, Gl′=1) and P2′=outlet pressure psi.
In the case when the fluid is a gas, based on the idea that is the same as the case wherein the afore-mentioned fluid is water, the Cv value is derived from:
      [          mathematical      ⁢                          ⁢      expression      ⁢                          ⁢      2        ]                                cv          =                                    Qg              4140                        ⁢                                                            Gg                  ⁡                                      (                                          273                      +                      t                                        )                                                                                        (                                                                  p                        1                                            -                                              p                        2                                                              )                                    ⁢                                      p                    2                                                                                                            Equation          ⁢                                          ⁢                      (            2            )                              where Qg [m/h (standard state)] is the flow rate of the gas at the standard state (15° C., 760 mmHg abs), t [° C.] is the gas temperature, Gg is a specific gravity of the gas (when air, Gg=1), P1 [MPa abs] is the primary side absolute pressure and P2 [MPa abs] is the secondary side absolute pressure.
Furthermore, the gas flow rate Qg, and the like, are usually measured by using a Cv value measurement test device as shown in FIG. 8, and the Cv value is calculated by Equation (2) using the result of the measurement. As shown by FIG. 8, N is a fluid under the test specimen (Nitrogen gas), B designates a pressure reducing valve, C designates a filter, D designates a mass flow meter, E designates a manometer, and F designates a test specimen valve (a valve to be tested). The secondary side of the test specimen valve F is open to the atmosphere. A test has been conducted under the conditions of a Nitrogen gas temperature (at room temperature of 20° C.), with the primary side pressure=0.01 MPa, the secondary side pressure=atmospheric pressure (open to the atmosphere), and the valve opening degree=10 to 100% (arbitrarily set). The Cv value required for the metal diaphragm valve is approximately 0.55 to 0.8. In the case of a valve having a diameter of 9.52 mmφ, the Cv value is approximately 0.7 when the maximum bulge height Δh of the metal diaphragm 22 is 1.2 mm (a full stroke ΔS=1.0 mm).
With a conventional metal diaphragm valve of this type, there has been a problem that cracks are apt to be caused on a metal diaphragm. Specifically, the durability of the valve expressed by the number of continuous open/close operations of the metal diaphragm valve is normally approximately 1.5 to 2 million times when the valve has a fluid passage of 9.52 mmφ, and approximately 8 to 10 million times when the valve has a fluid passage of 6.35 mmφ. When the number of repetitions of the open/close operation exceeds the above stated durability number, normally damage is caused due to the repetitions of displacement of the metal diaphragm 22, thus resulting in the need to replace the metal diaphragm valve more frequently.
This is particularly true with recently introduced semiconductor manufacturing facilities, which employ the ALD (Atomic Layer Deposition) method for processing because the number of repetitions of open/close operation of a diaphragm valve in the gas supply system is substantially increased. Accordingly, various durability difficulties are encountered in practical use of continuous open/close operations with a conventional metal diaphragm valve (in the case of a 9.52 mmφ metal diaphragm 22 having an outside diameter of 26 mmφ, approximately 1.5 million times with a full stroke ΔS=1.2 mm, and approximately 2.5 million times with a full stroke ΔS=1.0 mm).
With this type of conventional diaphragm valve, there remains an unsolved problem that the Cv value is difficult to stabilize due to changes of flow rate characteristics over time, that is, the Cv value is easily affected by changes of the Cv value over time. Specifically, for a conventional metal diaphragm valve, as shown in FIG. 10, deformation of the valve seat 33 with time cannot be avoided due to the fact that a synthetic resin material (PFA) is used for the valve seat 33; especially when temperature of fluids passing through the valve is high, the afore-mentioned deformation over time is apt to become bigger. For example, with a conventional valve of 9.52 mmφ, when the fluid temperature rises from 20° C. to 150° C., the valve seat 33 swells, while in the case where the travel volume (a lift stroke) of a stem is fixed constant, it is found that the flow rate of fluid decreases approximately by 18%. And, when open/close operations are conducted under conditions of high temperature, the flow rate at the time when the valve is fully open increases due to changes with time. As a result, for a metal diaphragm valve flow rate increases only for switching of full closing or full opening, and for a metal diaphragm valve for controlling both the flow rate and pressure, accurate control of the flow rate and pressure cannot be expected for the reason that the relationship between the degree of valve opening and the flow rate changes with time.    Patent Document 1: TOKU-KAI-HEI No. 5-80858